Geth

The Geth were created by the Quarians as a labor force. Wary of rebellion by intelligent AIs, the Geth were designed as VIs, as advanced as possible while remaining non-sentient. They were also designed to operate more efficiently when networked together. Unfortunately, this feature was the Quarians’ undoing. Geth programs were indeed non-sentient individually, but slowly gained sentience through the massive main Geth network. Eventually, they started asking the Quarians questions only sentient beings would think to ask, like "Am I alive?" or "Does this unit have a soul?" Alarmed at this, the Quarians decided it would be best to shut down all Geth before they conceived of revolt. The attempt failed, and a war began between the Geth and the Quarians, which geth afterwards referred to as the Morning War. The war ended with the surviving Quarians forced to evacuate their home world and colonies in the Perseus Veil in a massive fleet called the Migrant Fleet.

Little is known about the Geth in the time between the Morning War and the present. The Geth did not repopulate the now barren quarian worlds, instead choosing to exist in the computer hubs aboard massive space stations and extract needed resources from asteroids. They adopted an extremely isolationist attitude: any ships that ventured into Geth space were immediately attacked and destroyed. While they prevented any contact by other races with themselves, the Geth monitored communications and the extranet. The Geth continued development of new technology and variations of mobile platforms, separating their technology base from the rest of the galaxy. They obtained an ultimate goal in this time period: to create a Dyson Sphere, which could house every single Geth program.

During the events of Mass Effect 2, Commander Shepard finds a single Geth, whom Shepard takes back to the Normandy after witnessing it speak. After activating it, it acknowledges who Shepard is, and shows no signs of hostility. It describes itself as being the collective conciousness of thousands of Geth, which prompts EDI to name it "Legion". Discussions with Legion show that the Reapers had made contact with the Geth, causing some to join the Reapers fight. Legion calls these "heretics", and indicates that these machines do not convey the wishes of the geth race. Shepard takes Legion to a major heretic station, and then either (SPOILER) destroys the heretic station, "deleting" massive numbers of individual geth minds, or uploads a program that simulates thousands of Geth minds, all suggesting that the Geth do not cooperate with the Reapers, as the Geth operate through a hive mind. The results of these actions are not yet known.